The economic models are "fun".
Consider the couch surfer, staying in one place too long will incur a cost but travelling is less costly.
The costs increase the longer you stay.
They expect you to pay for food and stuff.
Not quite sure how to deal with the temporal issues.

I made a diagram for **Cost** that I think helps the discussion.


Also, see [Exercise 22 - Chapter 2](

Where the \\( X \\) and \\( X_{neg} \\) have the conventional meaning for \\( \le \\) and \\( \ge \\) while the \\( X^{op}
, X_{neg}^{op} \\) have the opposite meaning.